Where are strings for "Font Color", "Theme Colors", "Standard Colors","B", "I", "U" ? Cold not find in String Resource Browser or directly in ActiproSoftware.Products.Ribbon.SRName.
Those are not part of the Ribbon product itself, but are specified in the samples. If you search in the MainControl.xaml for the document editor demo, you can see them.
For the MiniToolBar we have the instance of that in the RibbonSamples/Common/RichTextBoxMiniToolBar.xaml file which gets used by the document editor demo. It gets displayed in the demo's RichTextBoxExtended.cs code.
But all the UI strings you mentioned for the MiniToolBar are either set in the RichTextBoxMiniToolBar.xaml file or in the ApplicationCommands.cs file if they are coming from commands (like for the ApplyDefaultForeground command).